In an ideal radio receiver, the antenna has a specific optimum length or specific parameters for every unique frequency to be received. The length of the antenna depends on the frequency to be tuned to. The radio front end is also known as a resonance circuit, and should be matching. Changing the frequency, without changing the values of components in the antenna resonance circuit, can lead to a limited mismatch in narrow bands (for example the frequency modulation (FM) band: 87.5 to 108 MHz).
The waveform length is calculated by dividing the speed of light by the frequency (in the following examples, the speed of light is taken to be 300000 km/s). Typically, the length of the antenna for a particular frequency is a quarter of the waveform length. Thus, considering the narrow FM band, the ideal antenna length for 100 MHz is 75 centimetres, the ideal antenna length for 87.5 MHz is 86 centimetres, and the ideal antenna length for 108 MHz is 69 centimetres. However, selecting an antenna length in the middle of this range does not result in severe complications in the reception of signals in the FM band, as the deviation in desired antenna length is approximately 25% across the full band.
However, changing the frequency without changing the values of components in an antenna resonance circuit for use in a wideband application can lead to large mismatch, as the following example shows.
In the medium wave (MW) band, the ideal antenna length for 1000 kHz is 75 metres, the ideal antenna length for 520 kHz is 144 metres, and the ideal antenna length for 1620 kHz is 46 metres. Thus, the deviation of approximately 300% across the full MW band is very severe for the MW band. A poorly tuned antenna that could be derived from the example will result in poor signal reception.
In many medium or long wave (LW) applications, a “ferroceptor” is used. The ferroceptor is a coil-shape magnetic antenna with a ferrite core. It has the advantages that it is a small antenna and is less sensitive to disturbances. However, it also suffers from the problem of poor tuning in wideband applications as outlined above.
It is known to tune antennas in several different ways. Since an antenna is a combination of a coil and one or more capacitors, it is possible to either change the inductance of the coil or the value of the one or more capacitors.
In wideband applications, such as MW and LW applications, a bank of capacitors can be provided. The values of the capacitors are chosen to cover the MW and LW bands as necessary, with regard to the ferroceptor inductance. In use, the antenna must be tuned to the frequency that the radio receiver is tuned to. That is, if it is desired to receive a signal at 700 kHz, the appropriate capacitor from the bank of capacitors must be selected. The tuning calculation is based on the ferroceptor value. For each new frequency selected, a new capacitance has to be calculated.
The inductance of a ferroceptor coil has a certain spread. The result of the capacitance calculation will therefore deviate from the ideal value. Consequently, this will be experienced by the radio end user as poor reception. Therefore, when an antenna is manufactured, the actual inductance value of the ferroceptor needs to be derived, and the inductance or capacitance changed accordingly. However, this tuning is complicated and consumes production time.
Therefore it is an object of the invention to provide for the alignment of an integrated capacitor bank to wideband frequency coverage used in radio receivers, which removes the need to adjust components external to the IC.